Many materials and combinations of materials, have been used as filtration media to remove solid or liquid particulate from fluid streams. The capabilities of such filter media are judged according to three main criteria: (1) the particulate removal efficiency (i.e. the ability of the filter media to capture and retain particulate), (2) the pressure drop for a given flow rate of fluid through the media (which is utilized as a measure of the power required to move the fluid stream through the media), and (3) the holding capacity (i.e., the total amount of particulate which can be retained by the media before the pressure drop becomes so great that the media must be cleaned or replaced).
Residential and commercial heating, ventilating and air conditioning systems (HVAC systems) deal with a wide variety of particulate, including dust, lint and pollen. Similar filtration systems are utilized in industrial spray painting booths to collect paint droplets from the exhaust air stream. Dust collection systems are also utilized in industrial settings to capture the by-products of manufacturing processes which are entrained in air streams. Obviously, the removal of such particulate in all of these settings are desirable for reasons of health, comfort and aesthetic appeal.
All filter media rely generally on either the attractive force between the filter media and the particulate, or "physical barrier filtration", to remove particulate from a fluid stream. The use of attractive forces includes electromechanical forces as well as chemical/adhesive forces. An example of electromechanical forces includes electrostatic filtration, wherein the natural electrical charge on a particulate, and the natural electrical charges on a filter media are such that the particulate is attracted to and retained on the filter media. An example of chemical/adhesive forces is present in the filtration of paint droplets from an air stream, wherein the paint droplets will adhere to the surface of the filter media. Physical barrier filtration utilizes filter media with openings sufficiently small to prevent particulate of a predetermined size (larger than the openings) from passing through the filter media.
Prior art "disposable" filters are designed to be built from low cost materials which may be affordably replaced when the filters become "dirty" (i.e. when the increased pressure drop due to retained particulate requires an undesirable increase in energy to move the fluid stream through the filter). Disposable filters are generally comprised of four constructions: (1) thick sheets (1/2 inch to 2 inch) of stabilized, high loft, non-woven fibrous media; (2) thin sheets (less than 1/16 inch) of stabilized non-woven fibrous media laminated to a metallic mesh material and then mechanically pleated; (3) thin sheets (less than 1/4 inch) of stabilized woven or non-woven fibrous media which has been sewn or glued to form a filter element which consists of multiple "bags"; and (4) stacked layers of expanded paper.
The stabilized, non-woven fibrous materials used for the first three above-described types of disposable filtration media are generally produced from natural and/or man-made fibers such as glass, cotton, polyester or polypropylene. The individual fibers may be either of a discrete staple length or continuous filaments. The stabilization methods for these fibrous media are generally either mechanical (such as needle punching), chemical (utilizing glues or binders), or thermal (utilizing plastic materials incorporated within a batting which are melted to bind the remainder of the fibers upon cooling of the melted material). These stabilized woven fibrous materials generally consist of layered sheets of large diameter man-made filaments loosely woven to form a fabric sheet.
The fourth construction type identified above typically consists of a plurality of layers of expanded paper. Each layer of this type of filter is created from a continuous sheet of paper which has been slit repeatedly, allowing the paper to be stretched in a fashion similar to an expanded metal screen. In this stretching process, each discrete slit widens, creating multiple openings through the paper. During the stretching of each paper layer, the strips of the paper, between slits, naturally twist to form a three-dimensional structure. Once expanded, each paper layer is heavily resinated to fix the paper in the expanded position. Layers of the expanded paper are then stacked atop one another resulting in a three-dimensional assembly having a tortuous path of openings through its thickness through which an air stream is directed.
Typically, the selection of a particular construction type is dependent upon a variety of factors, including cost requirements and use of the filter structure.
Since the present invention was first developed with a view towards use in a paint booth exhaust system, the problems associated with prior art filters in this setting will be more specifically addressed. The first decision to be made in filtration systems for paint booths is the type of filter structure to be utilized. Expanded paper filters are typically not effective barrier filters because of the large individual openings through the expanded paper filter. However, expanded paper filters can be effective in paint arrestance applications, because of the adhesive nature of the paint droplets. The contact of a paint droplet with the surface of the paper as an air stream proceeds through a filter, causes the droplet to adhere and be retained in the filter.
The main drawback with paper filters in paint arrestance applications lies in the fact that paint droplets passing through the filters exist in a very large range of sizes. Depending upon the size of the droplet and the type of paint, the paint droplets can dry and lose their adhesive qualities before contacting the filter media. In such a case, the solid paint particulate will not adhere to the paper, but rather will "bounce" through the filter media as it is pushed by the air stream moving through the filter. In an attempt to overcome this particular problem, most prior art paper filters utilize a thin layer of high loft non-woven batting as a final filtration stage, to capture dried paint droplets. The use of a high loft non-woven batting for the final stage of an expanded paper filter differs from the present invention in that the final layer of paper actually reduces the usable surface area of the high loft because the intimate contact between paper and high loft reduces the entrance surface area to the high loft batting. The main advantage of utilizing an expanded paper filter for paint arrestance, is in the large size of the openings through the paper, and the tortuous path taken by the air stream through the filter media. The large openings allow for the retention of significant quantities of paint particulate before the opening becomes overly restrictive due to paint accumulation. This restriction of the opening size increases the pressure drop through the media, thereby increasing the energy required to move air through the filter media. The tortuous path increases the possibility that paint droplets will contact the paper so as to adhere to the filter material.
While the expanded paper filter provides advantageous use in the area of paint arrestance, fibrous non-woven filter media are more adaptable to a wide variety of particulate filtration applications. In the production of non-woven battings from manmade fibers, the denier (the relative diameter) of the fibers may be chosen so as to define the size of the effective openings through the batting and thereby the effectiveness of the barrier filtration characteristic of the filter. Thus, the larger the denier of fiber utilized, the larger the effective opening sizes through the batting.
In determining an appropriate opening size for filter media, the characteristic of "surface loading" must be considered. Because the density of the particulate within the air stream is greater as the air stream enters the surface of the media, this entrance surface will generally "load" much more quickly than locations deeper within the filter. This loading obviously restricts the opening size and thereby increases the pressure drop of the filter media. Because of the loading of this surface, the filter media will require replacement (due to the increased pressure drop at the entrance surface) well before the full extent of the media has been utilized in capturing a particulate from the air stream.
It can therefore be seen that a compromise must be made between larger opening sizes (which provide lower pressure drop, greater holding capacity, and less surface loading) and smaller opening sizes (which provide increased particulate removal efficiency through a greater range of particulate size). While the fiber size may be adjusted as part of the "compromise", additional methods have been utilized to enhance the holding capacity of filter media without compromising the removal efficiency. Four general methods have been utilized in the prior art: (1) pleating of the media; (2) sewing or gluing the media into multiple "bags"; (3) multiple stage filters; and (4) mist separators.
In the first method, the filter media is pleated so as to increase the surface area of the filter element while retaining a small opening size. Typically, a thin metal mesh is laminated to the media to form a product which is mechanically pleated into an "accordion" shape. There are several drawbacks to this method. First, there is an increased cost in view of the metal material utilized and the lamination/pleating steps. Second, safety risks increase during the handling of the metal mesh due to the very sharp edges of the mesh. An increase in disposal and recycling problems are created by the combination of metal and otherwise disposable fibrous products. Finally, there is a lack of tensile strength in this type of filter media.
In order to utilize the pleated material described above, it must be adequately supported by an external frame. Otherwise, any application of tensile forces perpendicular to the pleat lines of the filter media would result in the flattening of the pleats. This lack of tensile strength prohibits the use of such filter media in any application which requires high tensile strength (such as on-roll commercial HVAC filtration in which the media is pulled from a supply roll, across the air duct work, and then wound up on a collection roll).
The second method identified above consists of sewing or gluing the filter media into multiple "bags" which are open at the entry plane of the filter and which extend downstream. Drawbacks of this method include the higher manufacturing costs of producing the "bags" and the higher initial cost in utilizing additional piping and physical space for this type of filter.
The third method utilized to improve the holding capacity of the media is to produce a multiple stage filter in which continuous, homogenous layers of non-woven fiber battings having different effective openness are laminated together. This creates a filtration media wherein the fluid steam is first presented to a more open layer having a larger denier, for removal of larger size particulate. The fluid stream then advances to layers of successively reduced openness to remove remaining smaller size particulate. The resultant filter is as efficient as that stage which has the smallest openings, but that final stage is not exposed to the full quantity of particulate and thereby minimizes the surface loading effect and extends the usable life of the filter.
The main drawback to the described multiple stage filter is that the entrance plane of the first layer and the interfaces between layers still act as entrance surfaces and are therefore subject to surface loading. In the case of paint arrestance filters where the paint droplets are of an adhesive nature, even a very open yet still continuous, fiber batting will capture most droplets at the entrance plane of the first batting causing surface loading of this batting. While the surface loading effect is minimized by the layered arrangement, it is still present.
The final method of enhancing a filtration media is described in U.S. Pat. No. 4,443,233 showing an improved mist separator. In this patent, a plate of metal is formed into a shape having raised and lowered areas. A fluid stream traveling through the plate follows tortuous changes of direction. During these changes of directions, large droplets (having greater momentum) will not change direction with the fluid stream, but rather will continue in a straight line until contact is made with the plate. In the use of such a filter to remove liquid particulate from an air stream, the liquid would condense on the plate, and the surface tension of the liquid would retain it on the plate.
Use of the mist separator filtration media for solid particulate filtration presents two major drawbacks. First, there would be a higher cost of materials due to the use of a metal plate, which thereby restricts the use of this media as a disposable filter. Second, the ability of the plate to retain solid particulate is minimal, because of the limitations of electromechanical attractions. After only a slight buildup of solid particulate occurs on the plate, the force of the fluid stream traveling past the plate would become sufficient to dislodge any additional buildup. In fact, the patent indicates that it is still necessary to provide a final stage of standard non-woven batting to capture smaller solid particulate.
In addition, this layer of standard batting would suffer severely from the surface loading effect since the presence of the metal plate would actually reduce the surface area of the batting due to its intimate contact with the batting.